Achieving high power efficiency is one of the most important goals in the design of the transmitter of a battery-powered wireless communications device, such as a cellular handset. The dominant consumer of power in the transmitter is usually the transmitter's power amplifier (PA). For this reason, efforts to improve the efficiency of the transmitter have focused in large part on ways to improve the efficiency of the PA.
The efficiency of a PA can be increased by operating the PA close to, or in, saturation. However, this approach is effective only if the signal applied to the PA has a constant envelope. Many existing or soon-to-be deployed technologies, such as Wideband Code Division Multiple Access (W-CDMA), High-Speed Packet Access (HSPA) and Long Term Evolution (LTE) cellular technologies, employ nonconstant-envelope signals in an effort to increase spectral efficiency. To maintain linearity and prevent the signal peaks of these nonconstant-envelope signals from being clipped as the signals are amplified by the PA, the signal levels must first be reduced before they are applied to the input of the PA and the PA must be operated in its linear region. Unfortunately, these requirements result in a substantial reduction in efficiency.
One approach that can be used to avoid the efficiency versus linearity trade-off of conventional linear-amplifier-based transmitter is to employ what is known as a polar transmitter. In a polar transmitter, the amplitude information (i.e., the signal envelope) is temporarily removed from the nonconstant-envelope signals, so that the polar transmitter's PA can be configured to operate in its nonlinear region where it is more efficient at converting power from the transmitter's power supply into RF power than when configured to operate in its linear region.
FIG. 1 is a simplified drawing of a typical polar transmitter 100. The polar transmitter 100 includes an amplitude modulator 102 configured in an amplitude modulation (AM) path; a phase modulator 104 configured in a phase modulation (PM) path; and a PA 106. The amplitude modulator 102 is configured to receive an AM signal AM(t) containing the signal envelope, while the phase modulator 104 is configured to receive a PM signal PM(t) containing the phase modulation. The amplitude modulator 102 operates to modulate a direct current (DC) power supply Vsupply according to amplitude variations in the AM signal AM(t). The resulting amplitude-modulated power supply signal Vs(t) is coupled to the power supply port of the PA 106. Meanwhile, the phase modulator 104 operates to modulate an RF carrier signal according to the phase modulation in the PM signal PM(t). The resulting phase-modulated RF carrier signal RFin has a constant envelope and is coupled to the RF input port of the PA 106.
Because the phase-modulated RF carrier signal RFin has a constant envelope, the PA 106 can be configured to operate in its nonlinear region of operation, where it is more efficient at converting power from the DC power supply Vsupply into RF power than is the PA of a conventional linear amplifier. Typically, the PA 106 is formed as a Class D, E or F switch-mode PA 106 operating in compression (i.e., “compressed mode”), so that the power supply port of the PA 106 is modulated according to amplitude variations in the amplitude-modulated power supply signal Vs(t). By modulating the power supply port of the PA 106, the AM in the original AM signal AM(t) is restored in the output signal RFout of the PA 106 as the PA 106 operates to amplify the phase-modulated RF carrier signal RFin.
Although the polar transmitter 100 does achieve a significantly higher efficiency compared to linear-amplifier-based transmitters, its utility can be limited in applications that require a wide range of controllable output powers. Some wireless communications applications require that the transmitter of a wireless communications device to be capable of controlling the transmitter average output power Pout over a wide dynamic range. For example, the W-CDMA standard requires the average output power Pout of the transmitter of a cellular handset to be controllable over a range of about +24 dBm to −50 dBm, i.e., over a 70 dB range.
The difficulty the polar transmitter 100 has in controlling average output Pout over a wide dynamic range is due in part to the difficulty in designing an amplitude modulator 102 that can control the amplitude of the amplitude-modulated power supply signal Vs(t) over a wide dynamic range. When the PA 106 is a switch-mode PA operating in compressed mode, the average output power Pout of the polar transmitter 106 depends on the amplitude (specifically, the square of the amplitude) of the amplitude-modulated power supply signal Vs(t) applied to the power supply port of the PA 106. Consequently, how effective the polar transmitter 100 is at controlling the average output power Pout depends, at least in part, on how effective the amplitude modulator 102 is at controlling the amplitude of the amplitude-modulated power supply signal Vs(t).
Various types of amplitude modulator circuits have been proposed for use in the polar transmitter 100. Most are designed with the goal of maximizing efficiency, so as not to sacrifice the efficiency gains achieved by being able to operate the PA 106 as a nonlinear PA. While some types of amplitude modulator circuits are highly efficient, most do not succeed in the ability of the amplitude modulator 102 to control the amplitude of the amplitude-modulated power supply signal Vs(t) over the range of amplitudes necessary to control the average output power Pout over the wide dynamic ranges required by standards. Controllability is particularly difficult when the average output power Pout to be controlled is low, such as at −50 dBm in the W-CDMA standard.
The ability of the polar transmitter 100 to control the average output power Pout over a wide dynamic range is also encumbered by the physical characteristics of the PA 106. As explained above, the PA 106 in the polar transmitter 100 operates as a switch, typically formed from a field-effect transistor (FET). In any FET, a stray capacitance is formed between the input and output of the FET (e.g., between the gate and drain). This stray capacitance provides a leakage path from the gate (to which the phase-modulated RF carrier signal RFin is coupled) to the drain (which serves as the output of the PA 106), which results in the PA 106 having a nonlinear response at low average output powers Pout.
The dynamic range limitation of the PA 106 results in spectral regrowth outside the desired or allocated transmission band. At low average output powers Pout, where distortion is most severe, the degree of spectral regrowth can be significant enough that compliance with standards is not possible. Hence, as a practical matter, the ability of the polar transmitter 100 to control average output power Pout over a wide dynamic range in compressed mode is limited by distortion at low average output powers Pout.
One approach that can be used to extend the range of average output power control of the polar transmitter 100 to lower average output powers Pout is to operate the PA 106 in what is referred to as “product mode” (or “multiplicative mode”) during times when the polar transmitter 100 is to transmit at a low average output power Pout, instead of operating the PA 106 in compressed mode. Such an approach is described in U.S. Pat. No. 7,010,276 to Sander et al. Unlike in compressed mode where the average output power Pout of the polar transmitter 100 is determined solely by the amplitude of the amplitude-modulated power supply signal Vs(t) applied to the power supply port of the PA 106, in product mode, the PA 106 is operated in its deep triode region where the average output power Pout is determined not just by the amplitude of the amplitude-modulated power supply signal Vs(t) but by the product of the amplitude-modulated power supply signal Vs(t) and the amplitude of the phase-modulated RF carrier signal RFin applied to the RF input port of the PA 106. The extra dimension of power control available in product mode affords the ability to extend the range of average output power control to average output powers Pout below that which can be controlled by the PA 106 when operating in compressed mode. However, this is accomplished at the expense of efficiency, since, as illustrated in FIG. 2, the PA 106 operates less efficiently in product mode than it does when operating in compressed mode.
Considering the drawbacks and limitations of conventional communications transmitters described above, it would be desirable to have methods and apparatus for generating and transmitting communications signals that are capable of controlling average output power over a wide dynamic range but which do not require a sacrifice in efficiency at low average output powers Pout.